A holographic reconstruction system in the sense of this invention preferably reconstructs moving three-dimensional scenes holographically in real-time with the help of video means.
The system comprises continuously controllable spatial light modulator means, which spatially modulate the light waves which are capable of generating interference with holographic information. Thanks to the effects of light diffraction, the modulated light waves reconstruct object light points, by way of local interference, said object light points optically reconstructing the three-dimensional scene. Light waves propagate in a directed manner from all reconstructed object light points towards the observer eyes, so that one or multiple observers can see these object light points in the form of the scene. This means that in contrast to a stereoscopic representation, a holographic representation realises a substitution of the object.
In order to achieve a satisfying quality of holographic representations, the observers should also be able to watch a reconstruction in a viewing space which is as large as possible. Depending on the distance of an observer, this requires a representation of a holographically reconstructed scene with a display screen whose size is characterised by a screen diagonal typical of today's television and video screens as background.
However, it is disadvantageous that a large holographic representation requires for large diffraction angles a much higher resolution of the light modulator means than would be necessary for a two-dimensional representation, as is described by the known sampling theorem. This makes extraordinarily great demands on the hardware and software resources of the holographic reconstruction system both as concerns the components for real-time provision of holographic information for encoding, and those for optical reconstruction of the scene.
Another known problem in a reconstruction system is an undisturbed propagation of the modulated light waves prior to generating interference. In order to reconstruct the object light points at the correct position in space, and with the correct light point values, at least a part of the interfering light waves must arrive simultaneously at all the positions at which object light points are to be reconstructed through interference. This means that each object light point requires spatial coherence among as many as possible of the interfering light waves.
Moreover, after the reconstruction of the object light points, the path lengths of all object light points of a wave which represent the three-dimensional scene must not exhibit any uncontrolled path length differences among one another as caused by controllable optical means.
In the description below, the term ‘optical axis’ denotes a straight line which coincides with the axis of symmetry of a reflecting or refracting optical element. Spatial light modulator means, which have been encoded by a hologram processor with holographic information of a three-dimensional scene, represent a ‘video hologram’. The interaction of a video hologram which is illuminated with coherent light with projection means causes a ‘modulated wave’ to be generated. The projection means define a ‘direction of propagation’ of the modulated wave, and this direction of propagation can be modified by ‘optical wave tracking’. If optical elements are disposed on the way to or if their effective direction is towards the video hologram, they will be referred to as ‘hologram-side’, and if they are disposed on the way to or if their effective direction is towards an eye position of an observer eye, they will be referred to as ‘observer-side’. A ‘visibility region’ describes a space which is disposed on the observer side at an eye position, and which represents the exit pupil of the system, and in which at least one observer eye must be situated for observing a reconstructed scene. If, as is the case in the present application, the modulated wave is tracked by wave tracking to the current eye positions, the ‘tracking range’ defines the space which embraces all eye positions for which wave tracking is possible. In the technical literature on the subject, such a projection system is also known as a projection system with eye tracking.
The applicant has already disclosed a number of holographic projection systems. For example, the international publication WO 2006/119760, titled “Projection device and method for the holographic reconstruction of scenes” describes a system which represents a holographic reconstruction of a three-dimensional scene in an enlarged manner.
This projection system takes advantage of a basic principle for holographic reconstruction, which has been described the first time by the applicant in their international publication no. WO 2004/044659, titled “Video hologram and device for reconstructing video holograms”, and which shall be explained with reference to FIG. 1.
In this embodiment, a plane light wave LW which is capable of generating interference and which is emitted by a modulator illumination means (not shown) illuminates all modulator cells of a transmissive spatial light modulator SLM which is dynamically encoded with holographic information of the scene. The encoded modulator thus represents a video hologram. A focussing lens L1, which realises a Fourier transformation of the light wave LW in its Fourier plane FTL, is disposed in front of the light modulator SLM, seen in the direction of light propagation. In a projection system, the light modulator SLM can modulate the incident wave LW with holographic information either in a transmissive grid mode, i.e. it can modulate a light wave which is capable of generating interference as it passes through the modulator, or it can serve as spatially controllable reflector. In either case, a modulated wave is created which reconstructs the object light points of the scene in the space in front of the Fourier plane FTL. The embodiment according to FIG. 1 shows only one object light point OP0 of the reconstructed scene.
Because of their matrix arrangement, the modulator cells modulate the wave spatially and equidistantly, thereby diffracting the light, so that a spatial frequency spectrum which comprises multiple diffraction orders m of the video holograms in different positions is created in the Fourier plane FTL. FIG. 1 shows a small part of the spatial frequency spectrum with the help of the example of the desired object light point OP0 in a selected diffraction order and the undesired reconstructed object light points OP+1 and OP−1 in adjacent diffraction orders. In the present embodiment, all modulator cells of the light modulator SLM are encoded for reconstructing the object light point OP0. This has the same effect as a lens CL which is controllable through encoding and which has a corresponding focal length.
In the projection system which has been disclosed in the international publication no. WO 2006/119760, a video hologram is again encoded on the light modulator. A spatial frequency filter which lies in the Fourier plane spatially filters one diffraction order out of the spatial frequency spectrum of the video hologram, and an optical projection system projects this diffraction order of the wave in an enlarged manner onto a focussing display screen.
The display screen focuses the modulated wave with the reconstructed scene in front of an eye position. An observer can watch the reconstructed three-dimensional scene behind the eye position in a visibility region.
Because the display screen projects reconstructed object light points of all diffraction orders in its focal plane, an observer would also see disturbing diffraction orders with one eye which is situated outside the visibility region, i.e. the other eye that is not provided with the content of the currently represented video hologram. The spatial frequency filter AP has an aperture which must not be larger than one diffraction order, and it thus selects one diffraction order of the modulated light.
The display screen can be a lens. However, as explained above, the diameter of the display screen must be very large compared with the size of the optical projection system, so that the display screen is preferably a concave mirror.
The reconstruction is fixed with the modulated wave, so that it will only be visible if at least one eye of the observer is situated directly in the visibility region behind the eye position, which is not physically visible. If the reconstructed scene is to be visible without any restrictions when the observer moves, a position controller must track the optical path of the entire modulated wave by way of optical wave tracking to the respective observer eye such that the tip of the reconstruction space is always close to the respective observer eye. For this, the exemplary projection system comprises an eye finder, known as such, which detects the current positions of the observer eyes and which controls with the help of the position controller the optical path of the modulated wave such that the latter is directed towards a desired eye position. In a system which provides a specific video hologram for each observer eye, the desired eye position is always the position behind which the observer eye lies that corresponds with the currently encoded video hologram. The current video hologram must not be visible to the other eye.
In the international publication no. WO 2006/119920, titled “Device for holographic reconstruction of three-dimensional scenes”, the applicant of this patent application discloses for example a holographic reconstruction device which requires the spatial light modulator means to be encoded specifically. In contrast to conventional video hologram encoding, where the holographic information is distributed across the entire modulator area, the applicant suggests to encode the information for each object light point of the scene only in a small region of the hologram which represents a sub-area of the entire encodable area of the light modulator means. The principle of encoding will be explained with reference to the FIGS. 2a and 2b, which show a detail of the holographic system according to FIG. 1. Both representations are limited to the modulation of the wave with holographic information. The further path of the wave is not shown.
If the light modulator SLM has a cell structure, corresponding additional object light points will unavoidably occur in other diffraction orders of the spatial frequency spectrum, represented in this example by the object light points OP+1 and OP−1 in the diffraction orders +1 and −1. Moreover, if the light modulator SLM carries the holographic information of a single object light point OP0 in all modulator cells, as shown in FIG. 1, the light will propagate in a wide angle after having reconstructing the object points OP+1, OP0 and OP−1, such that light from adjacent diffraction orders will always overlap in the Fourier plane FTL and all positions in the Fourier plane FTL will always also receive disturbing light portions from the undesired object light points OP+1 and OP−1, which cannot be removed by spatial filters. In order to avoid this drawback, the area of the encoded lens function CL must be adapted to the position of the object light point OP0 in space when encoding the light modulator SLM.
FIG. 2a shows an embodiment illustrating the encoding principle, with a detail of the holographic system according to FIG. 1. In this embodiment, the spatial light modulator means are encoded with a lens function CL whose surface area is adapted to the position of the light point in space. The surface area is reduced depending on the position of the object light point OP0 in space such that only light which reconstructs the object light point OP0 of the selected diffraction order passes the aperture Δx of a filter AP. In contrast, the filter AP blocks the light which reconstructs the undesired object light point OP+1 of the adjacent diffraction order.
FIG. 2b shows the encoding process for a section of a three-dimensional scene 3DS with the help of few object light points OP1 . . . OPn, said encoding process avoiding the light of adjacent diffraction orders to be overlapped using realisable spatial light modulator means. A computer-aided system controller (not shown) which controls the reconstruction process for all systems encodes with the help of a hologram processor for each single object point OP1 . . . OPn a separate lens CL1 . . . CLn of a multitude of adjacent modulator cells which, depending on its position in the reconstructed scene, lie in a confined region of the hologram of the light modulator area, such that all reconstructed light points in the Fourier plane FTL only emit their light to the used diffraction order. This prevents the light of adjacent diffraction orders from being overlapped, and undesired reconstructed object light points in unused diffraction orders from being visible through the exit pupil of the projection system.
In contrast to conventional holographic display devices, with this encoding method only the corresponding regions of the hologram carry the holographic information which is required for reconstructing the individual object light points of the scene. Only such object light points are encoded which must be visible from the current eye position in the visibility region, which is limited in its extent. This considerably reduces the computational load for encoding.